In my prior U.S. Pat. No. 3,333,967, a method is disclosed for preserving mature but less than fully ripe fruit which produce ethylene and are ripened thereby, using hypobaric conditions of about 100 to 400 millimeters of mercury (mm Hg) absolute pressure in nearly water-saturated, moving air to facilitate the diffusive escape of ethylene from the commodity without loss of water therefrom. This method was first discovered to give useful results on a laboratory scale, and later under favorable commercial conditions on a larger scale, with non-ripe, mature fruit such as avocados, limes, and especially bananas. Upon later study and research I discovered that hypobaric, i.e. low absolute pressure, storage is valuable for reasons in addition to promoting the diffusive escape of ethylene from stored commodities, and that consequently the method is applicable to a wide variety of metabolically active matter other than that which is influenced by ethylene. In some instances, for example meat, it presently appears that the efficacy of the method is due to a reduction in oxygen partial pressure which attends the pressure reduction, as well as to enhanced diffusive escape of volatile off-odors, but with other commodities more is involved. For example, the foliage of chrysanthemums produce large quantities of ethylene and is not affected by the gas, yet it responds well to hypobaric but not low-oxygen, atmospheric pressure storage. I also learned how to overcome several problems and difficulties which had restricted the use of the hypobaric method to pressures higher than the 100 mm Hg. limit described and claimed in my U.S. Pat. NO. 3,333,967.
It is known as in Bonomi's Br. Patent No. 822,904 that at atmospheric pressure, many forms of metabolically active matter ferment and are spoiled by the accumulated waste products of anaerobic respiration if continuously exposed to less than 3% oxygen. This happens to be equivalent to the oxygen partial pressure in air at 100 mm Hg. absolute pressure. Bonomi teaches only superatmospheric pressures between about 832-905 mm Hg. absolute and subatmospheric pressures between about 687-650 mm Hg. absolute: quite remote from my hypobaric method.
Heat exchange is so limited by the decreased heat capacity of air at pressures lower than 100 mm Hg. that it is not possible to cool a commodity and maintain it at a uniform temperature in dry air under these conditions. Indeed, at some low pressure which is unpredictable because it is determined in part by the geometry of the apparatus, the Dewar effect sets in and prevents all conductive heat transfer. I have discovered that the Dewar effect does not influence heat transfer in a commercially sized hypobaric trailer at pressures as low as 8 mm Hg., that conductive heat exchange can be kept at a satisfactory value at pressures lower than 100 mm Hg. by saturating the atmosphere with water vapor, that certain of the deleterious effects of too low an oxygen partial pressure can be obviated with advantage by periodically cycling the pressure back to atmospheric, and that fermentative waste products, being volatile compounds can be in part removed under hypobaric conditions.
These discoveries and improvements have enabled me to use the hypobaric process at pressures lower than 100 mm Hg., and thereby to increase its efficacy with certain commodities and extend its utility to other commodities. I have found that at pressures lower than 100 mm Hg., and especially below 50 mm Hg., even though the atmosphere is kept fully saturated with water vapor, commodity desiccation may occur because the commodity respires and thereby is slightly warmer than the surrounding atmosphere. The higher temperature causes the vapor pressure of water in the commodities to be greater than that in the surrounding air. At a low pressure the rate of diffusion of water vapor is so enhanced that the slight tendency for water movement from the commodity to the atmosphere, created by the vapor pressure differential, is greatly magnified. I learned that the use of certain water retentive plastic wraps suffices to prevent desiccation under these conditions, but the atmosphere still must be kept saturated with water, for the rate of passage of water vapor through the wraps is enhanced considerably when the pressure is lowered, thus rendering these moisture barriers far less efficient than if they are used at atmospheric pressure.
Another problem which becomes increasingly important as the pressure is reduced, especially below about 100 mm Hg., is evaporative cooling of the humidifying water. Upon enlarging the size of the hypobaric storage chamber to large commercial proportions, I learned that, because of this evaporative cooling effect, the said method and means disclosed in my U.S. Pat. No. 3,333,967 under various unfavorable conditions could fail to provide or maintain the high relative humidity that presently seems important for successful operation without a much reduced air through-flow.
I have now discovered how to use and profit by, and avoid deleterious consequences of, the refrigeration effect incident to free air expansion and water evaporation when air is bubbled through a body of humidifying water which is relatively smaller in relation to the whole storage space than was the body of water in relation to the size of a conventional laboratory vacuum vessel. While this cooling, often or sometimes, is desirable to lessen the work of, or eliminate other ways and means of cooling the chamber, refrigerating by evaporation of water runs counter to the objective of creating and maintaining high humidity. As the water cools, its vapor pressure is lowered and it tends to add progressively less water vapor to the incoming air so that the relative humidity in the chamber is reduced. In extreme cases, such as storage conditions near 0.degree.C, the water of the humidifier can freeze because of evaporative cooling.